What Can Be The Cause For The Pimples On The Stomach?

I have fat filled pimples on my upper stomach. I started having this since my puberty at around age 12 ( Now I am 28). When I touch them they are hard and bumpy, but sometimes ( not so often) some become soft and I can squeeze them. Once I squeeze them, there is just pure fat coming out with some small hairs. The pimples are not itchy and they do not bother me at all. But obviously they are not nice to look at. I was wondering what could be the cause of me having this kind o skin problem.